# intro to inductors question

Discussion in 'Homework Help' started by dayman, Sep 15, 2013.

1. ### dayman Thread Starter New Member

Jul 29, 2013
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so I have an opportunity to correct a Quiz from earlier this semester and Im stumped as to how to solve this question,

2. ### WBahn Moderator

Mar 31, 2012
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Q1) What are the continuity requirements that must be met from just before the switch is closed until just after it?

Q2) What does the answer to Q1 then require with regards to the voltage across each of the components in the circuit just after the switch is closed? Remember, it may impose a constraint on one component and not pose any constraint on another.

Q3) Given the answers to Q1 and Q2, what is the voltage across the remaining component(s) once you have taken KVL into account?

3. ### vk6zgo Active Member

Jul 21, 2012
677
85
You shouldn't need to ask us--Revision is a better idea!

4. ### dayman Thread Starter New Member

Jul 29, 2013
21
0
current lags behind voltage by 90 degrees in an inductor..

so, again bc im not sure

its 12 volts through both the instant the switch is closed?

5. ### #12 Expert

Nov 30, 2010
16,665
7,313
Second one today...volts aren't "through" a component, current goes through a component.

You must know one of your answers is wrong because the 12V has to show up somewhere, but your answers say it doesn't show up anywhere. If there is a time consideration anywhere, where is it?

6. ### dayman Thread Starter New Member

Jul 29, 2013
21
0
it takes 5 time intervals for the inductor to charge to its full current, so at the instant that the switch is closed the voltage would be at it's max potential in the inductor.

zero voltage across the resistor

please tell me if im wrong?

7. ### #12 Expert

Nov 30, 2010
16,665
7,313
Close. The maximum potential is on the left end of the inductor, not IN the left end of the inductor.

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8. ### dayman Thread Starter New Member

Jul 29, 2013
21
0
thank you...

you wanna tutor me? lol

thank you though....this course and the semiconductors course have been tough

9. ### WBahn Moderator

Mar 31, 2012
18,085
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You are falling into the trap of just memorizing rules and formulas without understanding what they mean and when they do and do not apply.

The "current lags behind voltage by 90° in an inductor" applies to systems in sinusoidal steady state operation. You are dealing with a transient operation.

Voltage appears ACROSS elements and the SUM of the voltages ACROSS all the elements in a closed loop must be ZERO.

Current travels THROUGH elements and the current through a set of elements in series must be the same.

So let's take it step by step:

In order to conserve energy, what must be true about the voltage and/or current in an inductor as a function of time? If you don't know, look in your Physics II text or in the introductory chapters of your electric circuits text.

10. ### WBahn Moderator

Mar 31, 2012
18,085
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That's because you appear to lack a lot of the fundamentals for one reason or another. Lot's of people do, so don't feel alone. But you have a choice to make -- do you address that weakness and build those fundamentals, or do you continue just trying to get by from one course to the next? If you do the latter, you will find each course getting progressively more difficult and may well run into a brick wall that you can't get past at some point. If you bite the bullet and go back and really learn the things you should have learned in the last class or two, you will find your classes much, much easier. The good news is that you are still pretty early in the game, so you haven't dug yourself too deep a hole yet. Plus, you now have some context upon which to hang your understanding so that things that fell into the category of, "Why do I need to learn this?", will now be in the category, "I can sure see why I need to learn this."

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11. ### #12 Expert

Nov 30, 2010
16,665
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Too true. Whenever I got only a "C", I would take the course over because the "C" grade make it glaringly clear that I did not "get" the material. You will fight all the way to the inevitable failure if you don't get some intuitive understanding of the basics.

Hang out here. It will help.

12. ### WBahn Moderator

Mar 31, 2012
18,085
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There were a couple of courses that I got a solid A in -- and felt good about my understanding of the material -- that I ended up spending time revisiting because of holes that reared their ugly head later on.